Project 1 will design antiviral fusion proteins and study and evaluate the mechanisms by which these fusion proteins give rise to an antiviral effect. The specific aims are: 1) To determine the source of the "escape inactivation fraction" of murine leukemia virus refractor to expression of Mo-MLV Gag-SN fusion proteins. The investigators will determine whether the fraction of virus that escapes inactivation by Gag-SN results from the emergence of resistant virus or from incomplete delivery of this antiviral protein to target cells in tissue culture. 2) To determine the safety and efficacy of Gag-SN fusion proteins against murine leukemia viruses in transgenic mice. The investigators will characterize the transgenic mice they have recently constructed that express MoMLV Gag-SN. Data on expression, possible toxicity, and especially protein against murine leukemia virus infection will be obtained. 3) To express HIV Gag- based antiviral fusion proteins and determine their effect upon the infectivity of HIV-1 virions (subcontract to Dr. John Kappes, University of Alabama). Based on experiments with the mouse retrovirus constructs, fusions between HIV-1 Gag and SN will be made during the last budget period that interfere with HIV-1; these consist of HIV Gag fused to dominant negative versions of the viral enzymes PR and RT. The mechanism of viral inactivation of the antiviral constructs will be investigated. 4) To identify and evaluate additional fusion protein properties for antiretroviral activity. The investigators will evaluate the potential of several new alternative antiviral protein fusion partners: RNase H, RNase A (human), Staphylococcal nuclease-calmodulin fusions, and integrase multimers. They will also attempt to deliver these antiviral proteins to HIV using cyclophilin A as the delivery vehicle (as an alternative to Gag). 5) To develop a high-throughput screen for anti-HIV fusion proteins for use in library screens. A screen will be set up to give a direct read out on antiviral effect, with the goal of screening thousands of potential antiviral constructs in parallel, and eventually of screening entire cDNA or peptide libraries efficiently.